1. CSc 110, Autumn 2016 Lecture 27: Sets and Dictionaries
Adapted from slides by Marty Stepp and Stuart Reges

2. Exercise
Write a program to count the number of occurrences of each unique word in a large text file (e.g. Moby Dick ).
Allow the user to type a word and report how many times that word appeared in the book.
Report all words that appeared in the book at least 500 times.
What structure is appropriate for this problem?

3. Dictionaries
dictionary: Holds a set of unique keys and a collection of values, where each key is associated with one value.
a.k.a. "map", "associative array", "hash"
basic dictionary operations:
put(key, value ): Adds a mapping from a key to a value.
get(key ): Retrieves the value mapped to the key.
remove(key ): Removes the given key and its mapped value.
my_dict["Juliet"] returns "Capulet"

4. Dictionary functions You can also use in, len(), etc.
my_dict[key] = value
adds a mapping from the given key to the given value; if the key already exists, replaces its value with the given one
my_dict[key]
returns the value mapped to the given key (error if key not found)
items()
return a new view of the dictionary’s items ((key, value) pairs)
pop(key)
removes any existing mapping for the given key and returns it (error if key not found)
popitem()
removes and returns an arbitrary (key, value) pair (error if empty)
keys()
returns the dictionary's keys
values()
returns the dictionary's values
You can also use in, len(), etc.

5. Maps and tallying
a map can be thought of as generalization of a tallying list
the "index" (key) doesn't have to be an int
count digits:
# (T)rump, (C)linton, (I)ndependent
count votes: "TCCCCCCTTTTTCCCCCCTCTTITCTTITCCTIC"
index 1 2 3 4 5 6 7 8 9
value
"T"
"C"
"I" 15 3
keys
values
key
"T"
"C"
"I"
value 15 3

6. items, keys and values
items function returns tuples of each key-value pair
can loop over the keys in a for loop
ages = {}
ages["Merlin"] = 4
ages("Chester"] = 2
ages["Percival"] = 12
for cat, age in ages.items()):
print(name + " -> " + str(age))
values function returns all values in the dictionary
no easy way to get from a value to its associated key(s)
keys function returns all keys in the dictionary